Method for carrying out a blind handover in an intersystem and interfrequency handover in mobile communication systems

ABSTRACT

The invention relates to a method for carrying out a so-called “blind handover” in an intersystem and interfrequency handover in mobile communication systems and builds on the fact that before the handover a mobile station is supplied by several base stations, i.e., in addition to the supplying base station, it can also receive signals from several other base stations. According to the invention a propagation time measurement is carried out at the air interface by the mobile station of the signals received from the base stations. The measured propagation times are transmitted to one of the base stations. Thereupon the residence site of the mobile station is determined on the part of the mobile communication network based on the transmitted propagation time measurement data. With the aid of a data base, subsequently and based on the determined residence site, at least one suitable base station for an intersystem or interfrequency handover is selected, and the data of the selected base station required for a handover are transmitted to the mobile station. Based on this information the mobile station can carry out the handover to the selected base station.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for carrying out a so-called ‘blindhandover’ in an intersystem and interfrequency handover in mobilecommunication systems, in particular in inhomogeneous network structuresof the participating mobile communication systems.

2. Description of the Prior Art

In the case of inhomogeneity of the different network structures, i.e.different frequency positions or coverage areas of the participatingnetworks, no ensurance for the successful completion of the HO can begiven in a so-called blind handover (HO).

The specification currently in effect provides that, for example, in ahandover between a UMTS layer and a GSM layer, per UMTS cell, one HOcandidate for a blind handover can be configured. In this case it isassumed that the coverage area of the target cell agrees with the sourcecell. Stated differently, the coverage area of the particular cell ofthe UMTS and of the GSM network are congruent at the site of thehandover. Otherwise the resulting situation is ambiguous. Theconsequence would be an HO error and therewith the risk of losing theconnection (call drop).

To avoid these problems in a blind handover, so-called dual mobilesubscriber terminal sets, i.e. mobile telephones with two separatetransmitting/receiving devices can be utilized, which are capable ofoperating simultaneously at two frequencies or in two mobile networks.

The signaling between subscriber terminal set and the participatingnetwork nodes, such as for example base stations, RNC (Radio NetworkController) and the mobile switching centers (UMSC) of the participatingmobile communication networks, exchanged during a handover, is notsubject matter of the method described here. It is therefore not furtherdiscusssed in the following.

SUMMARY OF THE INVENTION

The invention aims to specify a method with which even blind handoverscan be carried out without high risks between the different layers ofmobile communication systems, even if these do not have a common networkstructure.

The invention builds on the fact that, before the handover, a mobilestation is supplied from several base stations. This means that, apartfrom the supplying base station, it can also receive signals fromseveral other base stations. According to the invention a propagationtime measurement is carried out at the air interface by the mobilestation of the signals received from the base stations. The measuredpropagation times are transmitted to one of the base stations.Thereupon, on the basis of the transmitted propagation time measurementdata, the mobile communication network determines the [temporary]residence site of the mobile station. With the aid of a data basesubsequently on the basis of the determined residence site at least onesuitable base station for an intersystem or interfrequency handover isselected, and the data of the selected base station, required for ahandover, are transmitted to the mobile station. In conjunction withthis information the mobile station can carry out the handover to theselected base station.

With this method the capability is given of unerringly carrying out suchHOs with a further developed blind handover, also referred to in thefollowing as blind handover advanced.

The described method leads to important advantages:

-   -   In the so-called UMTS compressed mode, requisite HO measurements        must be initiated, which, depending on the situation and number        of HO candidates, make several GAPs (General Access Profile)        necessary. In a blind handover according to the invention no        time is lost.    -   Through the mechanism of the Compressed Mode (CM), additional        interference is generated in the network. In turn, this means a        reduction of capacity. In contrast, through the described method        an increase of capacity is attained, since no CM is necessary.    -   Through the method, other mobile communication services, which        require place of location information of the subscriber, can be        implemented without major additional engineering efforts.    -   The method functions within as well as also outside of buildings        without additional GPS.    -   The terminal sets do not need to be equipped with GPS or be        implemented as dual terminal sets (with two        transmitting/receiving units) and therefore can be produced more        cost-effectively.

Advantageous embodiments and further developments of the invention willbe evident to those skilled in the art.

To determine the site information the subscriber terminal set mustmeasure the [signal] level conditions in its own cell and at least inone or two further cells. In addition to the signal levels, the signalpropagation times at the air interface are also measured. Should thisactually not be required, due to the current level conditions of thesupplying cell, the terminal set by necessity has to be requested tocarry out this measurement. This can, for example, be performed therebythat to the terminal set specifically other supply level thresholdvalues are transmitted which compel a measurement or that the parameterof the network are set at the outset such that performance of thesemeasurements becomes mandatory.

The information of the signal propagation times generated in this manneris transmitted to the network. To be able to utilize this informationfor a blind handover, the layer in which the potential target cell islocated must previously be analyzed for the base station providing thebest supply, thus the best server has to be determined. This can becarried out in different ways. For one, the coverage area of the bestserver can be determined with appropriate methods and, for another, thiscan be done from available measuring data. The best servers determinedin this manner can be assigned at each point via [the geometry of] thepolygon.

The coordinates of the terminal set are subsequently compared with thebest server data base and in this way the appropriate target cell isselected. This target cell is subsequently transmitted through an HOcommand to the terminal set and therewith the blind handover advanced isspecifically completed.

BREIF DESCRIPTION OF THE DRAWING

An embodiment example of the invention will be explained in furtherdetail with reference to the drawing.

FIG. 1 depicts by example a section of the cell structures of twosuperimposed mobile communication networks, for example a UMTS networkand a GSM network.

DECRIPTION OF A PREFERRED EMBODIMENTS

The UMTS network comprises a multiplicity of radio cells 10–14 suppliedwith radio signals by a multiplicity of fixedly installed base stations20, 23, 24. The GSM network similarly comprises a multiplicity of radiocells 1–7 supplied with radio signals by a multiplicity of fixedlyinstalled base stations 20–22. The UMTS and the GSM network have, forexample, the place of location for base station 20 in common.

A mobile station 30 is located within UMTS cell 10 and is supplied withradio signals by, for example, base station 24. The mobile station 30intends to carry out a blind handover in a suitable radio cell of theGSM network.

According to the invention, for this purpose, first, the residence siteof the mobile station 30 must be determined.

Through a suitable application, the terminal set is requested to measurethe supply level and the quality of the base station 24 and of theneighboring UMTS base stations 20, 23. From the terminal set [sic] 30the appropriate base stations 20, 23, 24 must herein be unambiguouslyidentified and the associated propagation times of the signal at the airinterface must be determined. This information of the neighboring cellsand of the own cell are sent as an information packet to a base station,for example 24.

The place of location of the terminal set 30 can consequently becalculated in the UMTS network from only two measured neighboring cellsand the own cell. This method does not depend on the residence site ofthe terminal set either within or outside of a building.

To determine the residence site of a subscriber terminal set withoutknowing the direction information, thus at least three base stations 20,23, 24 are necessary, whose precise place of location is known. Based onthe measurements of the propagation time of the signals between theterminal set and each of the base stations, circular rings can becalculated which define the distance range of the terminal set from theparticular base station. In the center of each circular ring is locateda base station. The common intersection point of the three circularrings is the residence site of the terminal set. The places of locationof the base stations are herein reference points, the place of locationcoordinates being available from the place of location data base of thenetwork operator.

In theory, the three circles intersect in one point. Under realconditions, this is impossible since the propagation time measuringprinciple must depend on the propagation conditions and the processingspeed of the signals in the microchip of the terminal set (chipfrequency). The path segments per measuring interval cannot bearbitrarily small.

This means in practice that the residence sites of the terminal set isdescribed by a surface of intersection. Therein the accuracy of theplace of location determination increases with the number of measuredbase stations. Through a chip frequency of, for example, 3.84 Mhz, thesmallest measuring interval a per chip can be calculated to bea=speed of light C/chip frequency f _(bit)=300*10⁶/3.84*10⁶=78 m

With modern terminal sets in practice very much better measuringaccuracies can be obtained.

The accuracy also depends on the receiver of the terminal set. Thereceiver must be able to resolve time intervals within the chipfrequency in order to yield results in the 10 m range. The commonsurface of intersection of the circles indicates the probable area ofthe [temporary] residence of the terminal set.

Since the terminal set does not have any information about the framesynchronicity of the participating base stations, the determinedresidence site of the terminal set will have additional measuringerrors.

To circumvent this, mainly two options are available:

-   -   The base stations are synchronized via a central clock or via        GPS time.    -   Through measurements by the base stations the degree of        asynchronicity with other base stations is determined and stored        in a matrix.

Based on the determined residence site of the mobile station 30, theradio cell or base station of the GSM network best suited for a handoveris determined with the aid of a data base available in the mobilecommunication network. In FIG. 1 this is for example the GSM basestation 20, which inter alia supplies the GSM radio cell 1.

In order for the mobile station 30 to complete an HO to the appropriateGSM target cell 1, it is necessary to inform it of the target cell orthe appropriate base station 20, after the local information (measuredvalues) of the terminal set has been evaluated. This can be carried outdirectly in the form of an HO command to the mobile station.

This leads to the fact that the functionality of the appropriate networknode, for example RNC, must be expanded to the effect that theevaluation of the measurement data yields a local information, therefromby a best server data base the best supplying base station of the targetcell is determined and made available to the terminal set and the basestations participating in the handover.

1. Method for performing a blind handover in intersystem andinterfrequency mobile communication systems, whereby a mobile station isadapted for being supplied with radio signals from base stations of afirst mobile communication system and base stations of a second mobilecommunication system, comprising the steps of: making a mandatoryrequest of the mobile station to perform a measurement of propagationtimes of the radio signals being transmitted via an air interface fromthe base stations of the first mobile communication system to the mobilestation by providing the mobile station with specific signal levelthreshold values that compel a position measurement, measuring ofpropagation times of the radio signals by the mobile station,transmitting the measured propagation times one of the first basestations of the first mobile communication system, determining on thepart of the first mobile communication system the residence site of themobile station on the basis of the propagation time measurement data,selecting, based on the determined residence site of the mobile stationand, with the aid of a data base at least one suitable base station ofthe second communication system for an intersystem or interfrequencyhandover, transmitting data of the selected base station required for ahandover from the first mobile communication system to the mobilestation, and performing the handover by the mobile station from a basestation of the first mobile communication system to the selected basestation of the second mobile communication system (20).
 2. Method asclaimed in claim 1, characterized in that by the mobile station (30)additionally the signal strength and/or the signal quality of the basestations (23, 24) are measured and transmitted to one of the basestations.
 3. Method as claimed in claim 1, characterized in that themobile station (30) during the handover changes the utilized radiofrequencies.
 4. Method as claimed in claim 1, characterized in that theeffective coverage range of the base station (24) supplying the mobilestation before the handover differs from the effective coverage range ofthe base station (20) supplying the mobile station after the handover.5. Method as claimed in claim 1, characterized in that the effectivecoverage range of the base station (24) supplying the mobile stationbefore the handover overlaps the effective coverage range of the basestation (20) supplying the mobile station after the handover.
 6. Methodas claimed in claim 1, characterized in that the precise residence siteof the mobile station (30) is determined by means of a GPS receiver. 7.Method as claimed in claim 1, characterized in that by means of acentral clock a frame synchronization is carried out between theparticipating base stations (23, 24).
 8. Method as claimed in claim 1,characterized in that discrepancies of the frame synchronization betweenthe base stations (23, 24) are determined, stored in a matrix andutilized for calculating the residence site of the mobile station (30).9. Method as claimed in claim 1, characterized in that during thehandover the mobile station (30) changes from a base station (24) of afirst mobile communication system to a base station (20) of a secondmobile communication system.
 10. Method as claimed in claim 2,characterized in that during the handover the mobile station (30)changes from a base station (24) of a first mobile communication systemto a base station (20) of a second mobile communication system. 11.Method for performing a blind handover in intersystem and interfrequencymobile communication systems as in claim 1, wherein said positionmeasurement comprises measurement of propagation times.
 12. Method forperforming a blind handover in intersystem and interfrequency mobilecommunication systems as in claim 1, wherein said position measurementcomprises the setting of network parameters for the network at theoutset.